An Account team is a group of users who work collectively on a aparticular Account in Salesforce. An Account in Salesforce is nothing but a customer who gives business.
Account teams are different than Opportunity teams. The team members in Account team usually consist of different members such as Account executive, Project manager, Support executive etc..
We can also automate the Account team creation using a process builder in Salesforce.
We can also customize Account teams for a better team selling, which is a new feature in salesforce.